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RCA-76 cap question - Printable Version

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RE: RCA-76 cap question - Mondial - 04-08-2014

You can go higher with the resistor. I would try a 10K first and if the hum is still excessive go to 22K.

Check the voltage on the cathode of the AVC tube and see if it varies as you turn the volume control. The volume control pot is fed bias voltage from the power supply through another similar choke L 17 with a resistance of 2300 ohms, so perhaps it is open too?

RE: RCA-76 cap question - Eric - 04-08-2014

L17 checks good at 2190 ohms I have an inductance meter but it only goes to 40H and just shows OL meter checks against a choke that I have at 11h so your right it must be pretty high.
Cathode to plate went to 1vdc and was blasting if I turn the power down to 60vac it plays and I show .3vdc could c20 2400 mmf be bad?
One of my previous voltage checks I had 4.5vdc at that point.

RE: RCA-76 cap question - Mondial - 04-08-2014

Consider that there should never be any positive voltage to ground on the AVC tube plate. I doubt there is anything wrong with C20 as it is a mica with very little voltage across it

The AVC tube cathode is always negative with respect to ground and the plate will vary from zero to some negative voltage depending on the volume control setting. Measuring with respect to chassis ground, the cathode will always be more or less negative. Unfortunately the schematic does not tell what the negative voltage should be but I would take a guess it would be around - 30 or -40 V at the choke L17.

RE: RCA-76 cap question - Eric - 04-09-2014

I will check L17 to see what I get.
But also came across this Hammond choke coil #155c 60H 2700 ohms I think this might work for L14?

RE: RCA-76 cap question - Eric - 04-09-2014

Ok checked L17 and I get 0 volts from when I turn it on to when it starts blaring

RE: RCA-76 cap question - Mondial - 04-09-2014

That Hammond choke sounds like it will work fine for L14.

Are you measuring the voltage at L17 to chassis ground? There should be some negative voltage from either of its terminals to the chassis ground. If you measure across the two terminals of L17 you will read near 0 volts.

RE: RCA-76 cap question - Eric - 04-09-2014

yes went from each side of the coil to ground and got 0vdc
also just want to verify c38 pos goes to ground and the neg goes to the center tap

I'm going to order that choke coil

RE: RCA-76 cap question - Mondial - 04-09-2014

Yes, the positive of C38 should go to ground and the negative to one side of L17. The center tap of the power transformer should connect to the other side of L17 and the field coil of the speaker L18.

Note the speaker has TWO field coils, L18 and L19. L18 should develop a negative voltage across it. What resistance do you read from the center tap of the power transformer to ground? Should be the same as across field coil L18.

RE: RCA-76 cap question - Eric - 04-09-2014

I get 2.8k from the center tap to ground.
Some where I have a component that is breaking down or a bad solder connection?
Right now it is playing and the vol appears to be functioning I get neg 110-120vdc on L17 where before I had 0. Other times it goes dead sometimes lots of popping and static. Not sure where to go from here?